Intermittent fasting (IF) has some serious benefits. This dietary approach, which simply refers to going without food for a certain period of time, naturally restricts calories to help you lose weight.

Studies show that IF promotes ketosis and thereby increases fat burn, boosts cognitive functioning1, reduces inflammation1, and may even boost longevity.

But while all of these perks are awesome, there's still one thing we're a little confused about: What actually breaks a fast?

Hate to break it to you, but "technically, consuming any calories breaks a fast," says Benjamin Horne, Ph.D., a genetic epidemiologist who has published research on the effects of intermittent fasting.

Even a few calories' worth of food can inactivate some of fasting's perks. According to Horne, some mechanisms behind fasting's benefits, like ketosis (which increases fat burn), remain active with the consumption of certain macronutrients; but others, like autophagy, may or may not remain active.

"In humans, it appears that autophagy does not remain as active when any food is consumed," he says.

But what if we're talking about a measly 2 to 5 calories in a cup of tea or coffee? This ultra-low-calorie territory is where things get a little tricky.

According to Horne, it likely needs to be a water-only fast to maintain the maximum benefit from autophagy.

Others aren't so sure we need to be quite this nitpicky, though. "I've heard good debates about whether coffee can break a fast. We don't have any good evidence to show either way," says Vincent Pedre, M.D., an integrative physician and gut health expert who frequently recommends intermittent fasting diets to his patients.

Of course, we know that an all-or-nothing approach isn't always warranted or sustainable—and that there are still plenty of benefits to be had in the territory that lies between a strict water-only fast and eating a full-blown meal. Some foods maintain many of fasting's benefits—while keeping you sane.

So, many experts say it's OK to consume certain caloric foods in small quantities to help you stick to an intermittent fasting plan.  

Case in point: To help patients stay happy and compliant with their fast, integrative physician Amy Shah, M.D., allows the consumption of 30 to 40 calories from sources that won't spike blood sugar, like coffee or tea with a splash of unsweetened almond milk.

"I call this 'dirty fasting,'" says Shah. "For me, I like to fast as long as possible with just water—that might be 13 or 14 hours of a water fast—then I'll have my tea with almond milk (the start of my 'dirty fast') and end at around 16 hours."

Even Horne agrees that some perks of fasting remain with minimal food intake: "One mechanism that is known to remain active when a small amount of food is consumed is ketosis—as long as you consume less than 50 grams of carbohydrates [in a day]," he says.

"Some effects, such as the impact of fasting on the gut microbiome, may be different depending on whether it's a water-only fast or a very-low-calorie diet with a small amount of food consumed, but both may provide some level of benefit when compared to eating a standard amount of food."

Integrative dietitian Ali Miller, R.D., appreciates a more flexible approach as well.

"A fast is broken with consumption of food or a caloric substance; however, many people who enjoy the benefits of fasting and want to incorporate it as a daily ritual may take a more flexible approach such as a 'fat fast' using coconut oil, MCT oil, grass-fed butter, or cacao butter blended into a warm liquid during their fasted window."

As mentioned above, coffee and tea have such minimal calories that it's hard to say if they reduce any of fasting's benefits—at most, these drinks may slightly reduce autophagy.

Interestingly, one animal study2 found that coffee actually induced autophagy in mice, but it's not clear if autophagy would have been greater in the absence of all calories or if the results would have even been the same in humans. 

What we do know: In many cases, coffee and tea can make sticking to your fast that much easier.

"Caffeine does have some benefits for fasting, such as appetite suppression. For some people, it can be the needle mover they need to stick with fasting," says Pedre. "But if you're drinking black coffee and notice you're not getting results, I would suggest reducing or eliminating it and see if that helps."

You've likely heard of people whose version of intermittent fasting involves consuming nothing but coffee blended with MCT oil (or grass-fed butter, ghee, coconut oil, etc.) until around noon.

This, which Miller calls a "fat fast," technically breaks your fast and reduces autophagy to some extent but can effectively curb cravings while maintaining some of fasting's key benefits.

"A fat fast would be supportive for improving satiety—thereby helping you hold off on eating a true meal—and aiding in body fat metabolism and ketone production in the morning, at a time when insulin levels are typically elevated from morning cortisol surge," says Miller.

Without a dose of fat, these elevated insulin and cortisol levels may potentially trigger hunger or cravings and prompt you to eat.

Bonus: A little fat in the morning can also help keep those bowels regular. Consuming a healthy form of fat during your fasting period may also be particularly beneficial if weight loss isn't your main goal.

"Low body fat levels can drive imbalances in hormones, so a 'fat fast' would be appropriate for people looking for moderate body composition changes or maintenance," says Miller.

For someone with more weight to lose, on the other hand, a pure water fast would be less likely to throw off hormones.

Another liquid that's frequently touted as "acceptable" during a fast is bone broth. Again, this technically breaks a fast, but depending on your goals, it can be a smart addition.

Even renowned intermittent-fasting expert Jason Fung, M.D., gives it a thumbs-up as a way to reduce hunger pangs and improve fasting compliance while delivering much-needed minerals and electrolytes.

Consuming water alone, especially if your fast exceeds 16 hours, can reduce electrolyte levels in the body, leading to potential complications such as low blood pressure, confusion, nausea, muscle spasms, and fatigue. (Of course, if you don't want to consume bone broth, Miller says you can simply add a couple of teaspoons of mineral-rich sea salt to your water.)

Bone broth, rich in gelatin and the amino acid glutamine, can also be particularly beneficial if you're looking to rebalance or heal the gut.

Some research, however, shows that glutamine (and protein in general) fuels a process called mTOR, which prevents autophagy. So, you don't want to overdo and continuously sip on bone broth (or coffee spiked with collagen powder) throughout your fasting hours.

Despite the fact that they don't contain calories, most of the experts we spoke with said zero-calorie sweeteners should be avoided during a fast—and pretty much all the time.

"I'm not a fan of artificial sweeteners at all," says Pedre. "Technically these have zero calories, so they would be 'legal' during a fast, but we've seen how they can disrupt gut balance and cause lots more problems. I would steer clear of them altogether, and if you need a sweetener, use organic 100% stevia sparingly during fasting hours."

Miller agrees, explaining that even though something has zero calories, it can still negate some of fasting's most significant benefits, including appetite control.

"Just because something is noncaloric doesn't mean it is free of metabolic influence," says Miller. "The taste of sweet impacts GLP-1 receptors on the tongue, which enhances insulin release—this is not ideal, as insulin has a negative impact on fasting and may drive blood sugar drops."

Ideally, intermittent fasting will help you curb cravings and reduce hunger over time due to its beneficial effect on insulin sensitivity. But if you're not feeling well during your eating hours, intense cravings can result, making you feel like you "need" that butter-laced coffee first thing in the morning.